This invention relates to a method and systems for combusting fuel to supply heat to boilers and superheaters of externally fired power plants.
Combustion systems currently used for externally fired power plants can be divided into two categories: systems in which air fed into the combustor is preheated and systems in which air is not preheated. A combustion system with air preheating allows for higher efficiency of the power system because the heat is released in a temperature range above the temperature. at which flue gases are utilized to preheat oncoming air, e.g., usually in a temperature range above approximately 700.degree. F.
Two known types of combustion processes include pulverized coal combustion systems (in which combusted coal is micronized into a thin powder) and fluidized bed combustion systems. Pulverized coal combustion systems are usually used only for burning coal (which, unlike other types of fuel, can be ground into thin powder). The thin coal powder is typically burned with at least 15-20% of excess air to provide complete combustion. The removal of heat is performed by special tubes surrounding the flame zone. In these tubes, the working fluid (usually water) is boiling, thus preventing the tubes from overheating and burning. The temperature of combustion typically is very high so that environmentally hazardous nitrogen oxide (NO.sub.x) gases are produced as a by-product. The high combustion temperature also prevents removal of environmentally hazardous sulfur gases, e.g., through limestone addition.
In fluidized bed combustion systems, different forms of solid fuel can be combusted. The fuel is usually fed into the combustion zone mixed with limestone; the purpose of the limestone is to remove sulfur gases. As in the pulverized coal system, the fuel is typically burned with at least 15-20% of excess air to provide complete combustion. Special cooling tubes which acquire heat in such combustors are located directly in the combustion zone, thus providing effective cooling. As a result, fluidized bed combustion systems can combust fuel at temperatures of approximately 1600.degree. F. At such temperature limestone absorbs sulfur gases and NO.sub.x gases are not produced from air. However, because the bulk of the heat is released by a single cooling of flue gas from 1600.degree. F. to approximately 700.degree. F., there is not enough released heat for reheating the working fluid in the power cycle, thereby decreasing the efficiency of the power system.